Why does my survey number incorrectly?

This has happened a few times. I insert a question right after question #50 and everything looks fine. Then it becomes #27 but it still is between #50 and #52. Even when I delete the new question and begin again, it does the same thing. I have also noticed large numbering gaps – for example, the questions go from 30 straight to 50 – is this just something that happens in the build mode? Thanks, Julie